Over the past several years, various studies have been conducted to measure the levels of toxic and related air pollutants in many different geographical areas. For example, a number of investigations have been made to measure ambient concentrations of HNO.sub.3, SO.sub.2, nitrates and sulfates which contribute to acidic deposition and acid rain phenomena in the environment. In order to increase the accuracy of these types of measurements, more precise sampling and analysis apparatus and procedures have been developed for measuring ambient concentrations of the aforementioned chemicals. For example, filter packs consisting of an inert filter followed by a treated filter have been developed to provide apparently reliable data for measuring sulfates and SO.sub.2.
However, measurement of HNO.sub.3 and nitrate compounds has proven to be very difficult owing to losses of HNO.sub.3 in the sampling systems, for example, in the inlet portions of the sampling apparatus, and owing to the difficulty in differentiating vapor phase HNO.sub.3 from HNO.sub.3 produced from the dissociation of NH.sub.4 NO.sub.3 during sampling procedures. The use of one or more annular denuders connected in series has been proposed by Possanzini et al, Atmos. Environ., 17, 2605 (1983), for sampling ambient concentrations of HNO.sub.3. The denuders are treated with chemicals such as sodium carbonate and citric acid so that most of the gaseous acids and bases can be absorbed onto the denuders. This allows the particles to pass through the denuders without undergoing chemical transformations or being deposited on the denuder surfaces by diffusion. However, it appears that the presence of larger particles in a sample effects the efficiency of the denuders owing to the difficulty in sampling and quantifying the larger particle fraction, particle re-entrainment and volatilization of large particle chemical components. Accordingly, there is a need for a means which separates larger size particles from the sample while retaining smaller size particles and gaseous components of the sample.